Ideomotor apraxia and cerebral dominance for motor control

Ideomotor apraxia is a symptom of left hemisphere damage. Patients with ideomotor apraxia commit errors when imitating movements with their left, non-paralyzed hand. This has been taken as evidence for a motor dominance of the left hemisphere. It has been hypothesized that the left hemisphere is dominant for internal preprogramming of skilled movements of either hand. We investigated the kinematics of movement trajectories of imitation of meaningless gestures. Group analysis confirmed that hesitant, feedback-controlled movement prevail in patients with apraxia, but analysis of single cases revealed the existence of kinematically normal movements leading to apractic errors. Enhanced reliance on feedback-control appears to be a compensatory strategy rather than the source of apractic errors. In a second study we explored the alternative hypothesis that patients with apraxia lack a general concept of the human body which is necessary to mediate the translation of a target position seen on the model into a target position on the patient's body. Imitation of movements was examined on oneself and on a mannikin. Patients with apraxia who made errors when imitating on themselves committed errors also when imitating on the mannikin. Taken together, both studies support the view that the source of errors in the imitation of gestures is to be sought at a conceptual level. This casts doubts on the alleged dominance of the left hemisphere for motor control.